(57) A process for preparing liposome, which comprises hydrating a liposome membrane component
material containing phospholipid and cholesterol at a temperature not lower than its
phase transition point, then adding an aqueous solution at a temperature not higher
than Tc and stirring the mixture. In this process, hydration is conducted at a temperature
not lower than Tc, whereas dispersion in an aqueous solution is conducted at a temperature
not higher than Tc. Therefore, even when a membrane component material having high
Tc is used, the aqueous solution is not exposed to high temperatures, and hence even
an aqueous solution containing substances unstable to heat can be used. Therefore,
this process enables preparation of liposome containing within the inner phase a physiologically
active substance liable to be denatured or deactivated, such as hormones (e.g., insulin,
hydrocortisone, etc.), various oncostatic agents, or enzymatic drugs, e.g., superoxide
dismutase.
Technical Field
[0001] The present invention relates to an improved method for preparing liposomes. More
particularly, it is concerned with a method for preparing liposomes by hydrating liposome
membrane-constituting materials at a temperature not lower than the phase transition
temperature (Tc), then adding an aqueous solution at or below the Tc followed by vigorous
stirring.
[0002] Liposomes are widely used in the field of medicine as a carrier for a variety of
physiologically active substances.
[0003] The method of the invention is particularly favorable for the production of the liposomes
into which a substance being unstable at higher temperatures is to be incorporated.
Background Technology
[0004] Liposomes are prepared by dissolving liposome membrane-constituting materials in
an appropriate organic solvent such as chloroform, distilling off the solvent from
the resulting solution to form a lipid film to which is then added an aqueous solution
of a drug followed by vigorously stirring to an aqueous suspension and then treating
the suspension with such a sonication. It is known that the preparation of liposomes
is necessary to be carried out at a temperature not lower than the phase transition
temperature (gel-liquid crystal phase transition temperature, Tc) and that at a temperature
lower than
Tc a homogeneous suspension will not be produced when the lipid is suspended in water
(Saishin Igaku (Modern Medicine) Vol. 37, No. 2, pp. 338-345). Thus, when dipalmitoylphosphatidyl-choline
(DPPC) which has a Tc of 41°C is employed as a membrane-constituting material, suspension
must be made at a temperature not lower than 41°C so that it is very difficult to
incorporate a substance extremely unstable at higher temperatures such as a biological
substance (for example, hemoglobin) into the inner aqueous space.
[0005] As a result of extensive studies we have found that liposome membrane-constituting
materials including phospholipids and cholesterol when hydrated at a temperature not
lower than the Tc form liposomes with an aqueous solution held even if liposome formation.,
that is, incorporation of the aqueous solution into liposomes is carried out at a
temperature not higher than the Tc.
[0006] When phospholipids are completely hydrated to form a lamella phase (lipid bilayers),
liposome formation readily occurs. If, however, phospholipids are mixed with water
at a temperature not higher than the Tc, water hardly enters the hydrophilic group
due to the hydrocarbon chains of phospholipid regularly arranged, so that the hydration
is difficult. Therefore, when phospholipids are mixed with water at a temperature
not lower than the Tc, preferably not lower than the Tc in powdery crystal, it is
conceived that the hydrocarbon chains of the phospholipids are in "melted" state,
and the hydration is easy. It is also believed that when phospholipids hydrated to
lamella phase are cooled to a temperature at or below the Tc, it will become gelled
state, which makes formation of liposomes difficult, but when cholesterol added, the
branched paraffin chain of the cholesterol will enter the central portion of lipid
bilayers resulting in increased fluidity at the central portion, and liposomes may
be formed readily from gel-like lipid thus formed. Thus, hydrated phospholipids containing
cholesterol are capable of forming liposomes at a temperature not higher than the
Tc so that substances being unstable at a higher temperature can be incorporated into
the inner aqueous space.
[0007] The present invention has been completed on the basis of the above-described findings,
the object of which is to provide a method for preparing liposomes allowing for production
of liposomes at a temperature not higher than Tc of the liposome membrane-constituting
materials.
Disclosure of the Invention
[0008] The above-mentioned object is achieved by a method for preparing liposomes according
to the invention as set forth below.
1) A method for preparing liposomes which comprises hydrating liposome membrane-constituting
materials including phospholipids and cholesterol at a temperature not lower than
phase transition temperature (Tc) of the membrane-constituting material with water
added, then adding an aqueous solution at a temperature not higher than the Tc and
vigorously stirring the mixture.
2) A method according to the item 1 wherein the phospholipids are hydrogenated natural
phospholipids or synthetic phospholipids composed of saturated fatty acids.
3) A method according to the item 2 wherein the hydrogenated natural phospholipids
or the synthetic phospholipids composed of saturated fatty acids are hydrogenated
natural lecithins or synthetic lecithins composed of saturated fatty acids.
4) A method according to any of the items 1 to 3 wherein the liposome membrane-constituting
materials include a negative charge-providing substance.
5) A method according to the item 4 wherein the negative charge-providing substance
is phosphatidic acid, dicetyl phosphate or saturated higher fatty acids.
6) A method according to the item 1 wherein the aqueous solution contains a physiologically
active substance.
7) A method according to the item 6 wherein the physiologically active substance is
hemoglobin.
[0009] The method of the invention is easily conducted by adding an appropriate amount of
water to liposome membrane-constituting materials containing phospholipids and cholesterol,
heating the mixture at a temperature not lower than the phase transition temperature
for hydration followed by addition of an aqueous solution and vigorously stirring
the mixture.
[0010] There is no particular limitation on the nature of phospholipids in the invention,
and any of those natural or synthetic phospholipids which form liposomes may be employed.
In view of the object of the invention, however, those which have a relatively high
Tc are used. Particularly preferable are hydrogenated natural lecithins or synthetic
lecithins composed of saturated fatty acids. These phospholipids are hardly apt to
form toxicant peroxides. As they are also stable to oxidation, they are especially
favorable when a substance unstable to oxidation is to be incorporated into the inner
aqueous space. The hydrogenated natural lecithins as mentioned above are hydrogenation
products of the lecithins obtained from a natural material such as egg yolk or soybean.
Degree of the hydrogenation is 30 or below, preferably 10 or below in terms of the
iodine value. Cholesterol which is effective for increasing fluidity of the phospholipid
membrane is preferably added in a ratio of 1 mole or more to 2 moles of the phospholipid.
In addition to the phospholipids and the cholesterol, known additives such as a negative
charge-providing substance for the prevention of agglutination (for example, phosphatidic
acid or sodium salt thereof or dicetyl phosphate) and an antioxidant (for example,
tocopherol) may be added as the liposome membrane-constituting material. These membrane-constituting
materials are preferably employed at the ratios of 0.2-1.0 parts by weight of cholesterol
and 0.1-0.3 parts by weight of a negative charge-providing substance to 1 part by
weight of phospholipids.
[0011] There is no limitation on the aqueous solution to be incorporated into the inner
aqueous space of liposomes, and aqueous-solutions containing various physiologically
active substances are used. In view of the features of the invention, however, physiologically
active substances that are deteriorated or inactivated at higher temperatures such
as hemoglobin, hormones, for example, insulin and hydrocortisone, anticancer agents
and substances of living body origin such as enzyme preparations, for example, SOD
(superoxide dismutase) are preferably used.
[0012] In carrying out the preparative method of the invention, first, membrane-constituting
materials are completely dissolved in an organic solvent such as chloroform followed
by removal of the solvent by such a means as reduced pressure or spraying of an inert
gas to form a homogeneous mixture. Subsequently, distilled water is added, and the
mixture is completely and wholly hydrated by such a procedure as kneading at a temperature
not lower than the phase transition temperature (Tc). Although the hydration is more
easily effected with more amount of the distilled water, the aqueous solution of a
drug will be diluted with the excess amount of water to result in, reduction in holding
the drug in liposomes. An adequate amount of the distilled water to be added is 1
milliliter per gram of the membrane-constituting materials. Then, the mass is maintained
at such a temperature as causing no inactivation of a drug to be held and mixed with
an aqueous solution of the drug followed by stirring. Further, the liquid mixture
is dispersed by such a means as a pressure emulsifier, French pressure cell or sonicator
while carefully observing the temperature. The operation is continued until an appropriate
particle size is produced.
[0013] The liposomes prepared as above are subjected to such a procedure as dialysis, gel
filtration, ultrafiltration or centrifugal separation to separate the free drug and
may also be suspended in an aqueous solution for specific applications.
[0014] The invention will be described in more detail below with reference to an example
and comparative examples.
Example
[0015] In 50 ml of chloroform were dissolved 2.90 g of hydrogenated egg yolk lecithin (Tc=45-60°C,
Tc
max=55°C), 0.73 g of cholesterol and 0.39 g of phosphatidic acid sodium salt (from egg
yolk lecithin), and the solution was placed in a 300-ml round-bottomed flask. The
chloroform in the flask was removed by a rotary evaporator followed by vacuum drying
to obtain a dried product of the membrane-constituting materials. To the product was
added 4.0 ml of distilled water, and glass beads were put in the flask, which was
shaken at 80°C (>Tc) for blending and hydration. The flask was then placed and cooled
in a water bath maintained at 4°C «Tc), tb which was added 25 ml of 30% (w/v) hemoglobin
solution. The mixture was well shaken to an almost homogeneous liquid, which was then
subjected to 10 continuous treatments by passing through a French pressure cell (manufactured
by Ohtake Seisakusho) at a pressure of 700 kg/cm
2 at 4°C. After addition of 200 ml of physiological salt solution, the mixture was
centrifuged (27000x g, for 30 min.) at 4°C. Precipitates thus obtained were suspended
in 50 ml of physiological salt solution, and centrifugal separation and re-suspension
were repeated until no color of hemoglobin was observed in the supernatant from centrifugation.
The finally produced liposome suspension colored pale red and was observed in visible
absorptiometry by a spectrophotometer (Model UV-300 manufactured by Shimazu) for a
spectrum characteristic of oxyhemoglobin. Degradation by oxidation was not observed.
A large number of spherical particles of approximately 1 µm or less in size were microscopically
observed.
[0016] Ratio of the amount of hemoglobin held in the final liposome suspension to that added
on preparation was 19.1% according to hemoglobin assay by the cyan methemoglobin method.
The hemoglobin-containing liposomes thus obtained are useful as artificial erythrocytes.
Comparative Example 1
[0017] A dried product of the membrane-constituting materials was obtained in the same way
as in the Example. To it was added 25 ml of 30% hemoglobin solution. The mixture was
shaken at 65°C (>Tc). The solution gradually colored brown and formed precipitates
which were considered to be degraded hemoglobin.
Comparative Example 2
[0018] A dried product of the membrane-constituting materials was obtained in the same way
as in the Example. To it was added 25 ml of 30% hemoglobin solution. The mixture was
shaken at 4°C (<Tc). The lipids were hardly hydrated and remained unchanged almost
in solid. Subsequently, the mass was dispersed at 4°C by means of a bath type sonicator.
Free hemoglobin was removed in the same manner as in the Example. The final liposome
suspension colored milk white with pale red tint. Examination of the visible absorptiometry
made in the same way as in the Example revealed that it had a slight absorption spectrum
characteristic of hemoglobin. A number of distorted particles of several µm in size
were microscopically observed. Ratio of the hemoglobin held in the final suspension
as determined in the same way as in the
Example was 2.3%.
Industrial Applicability
[0019] According to the invention, there is provided a method for preparing liposomes which
comprises hydrating liposome membrane-constituting materials including phospholipids
and cholesterol at a temperature not lower than the phase transition temperature and
then adding an aqueous solution at a temperature not higher than the Tc followed by
stirring.
[0020] In the present invention, whereas the hydration was carried out at a temperature
not lower than the Tc, the dispersion into aqueous solution was carried out at a temperature
not higher than the Tc so that the aqueous solution remains unheated even when membrane-constituting
materials with a high Tc are employed and, therefore, aqueous solution containing
a heat-unstable substance can be used.
[0021] According to the invention, there can be prepared liposomes which contain in the
inner aqueous space a physiologically active substance liable to degradation or inactivation
such as hemoglobin, hormones, e.g., insulin and hydrocortisone, anticancer drugs,
or enzymatic preparations, e.g., SOD (superoxide dismutase).
[0022] Moreover, when hydrogenated natural phospholipids or synthetic phospholipids composed
of saturated fatty acids are used as the phospholipids in the invention, not only
such phospholipids have a high Tc and are very stable to oxidation of the membrane
constituents, but also the inner aqueous space will hardly be influenced by the oxidation.
Accordingly, the method is favorable for preparing liposomes containing a substance
liable to oxidation.
1) A method for preparing liposomes which comprises hydrating liposome membrane-constituting
materials including phospholipids and cholesterol at a temperature not lower than
phase transition temperature (Tc) of the membrane-constituting material with water
added, then adding an aqueous solution at a temperature not higher than the Tc and
vigorously stirring the mixture.
2) A method according to Claim 1 wherein the phospholipids are hydrogenated natural
phospholipids or synthetic phospholipids composed of saturated fatty acids.
3) A method according to Claim 2 wherein the hydrogenated natural phospholipids or
the synthetic phospholipids composed of saturated fatty acids are hydrogenated natural
lecithins or synthetic lecithins composed of saturated fatty acids.
4) A method according to any of Claims 1 to 3 wherein the liposome membrane-constituting
materials include a negative charge-providing substance.
5) A method according to Claim 4 wherein the negative charge-providing substance is
phosphatidic acid, dicetyl phosphate or a saturated higher fatty acid.
6) A method according to Claim 1 wherein the aqueous solution contains a physiologically
active substance.
7) A method according to Claim 6 wherein the physiologically active substance is hemoglobin.